1. Field of the Invention
The present invention relates to a golf ball and more particularly to a dimple pattern of the golf ball.
2. Description of the Related Art
A golf ball has approximately 300 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight of the golf ball to accelerate transition of a turbulent flow at a boundary layer, thereby causing a turbulent flow separation (which will be hereinafter referred to as a xe2x80x9cdimple effectxe2x80x9d). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backwards so that a pressure resistance is reduced, resulting in an increase in a flight distance of the golf ball. Moreover, the acceleration of the transition of the turbulent flow increases a distance between upper and lower separating points of the golf ball which is caused by backspin. Consequently, lift acting on the golf ball is increased. Accordingly, a dimple pattern capable of easily accelerating the transition of the turbulent flow, that is, a dimple pattern capable of better disturbing an air stream is more excellent aerodynamically.
A regular polyhedron or a quasi-regular polyhedron (which will be hereinafter referred to as a xe2x80x9cpolyhedron) is often used for the dimple pattern. More specifically, a polyhedron inscribed on a sphere is supposed, and sides of the polyhedron are projected on a spherical surface by rays irradiated from the center of the sphere onto the spherical surface, thereby forming a comparting line. The spherical surface is comparted by the comparting line. Thus, the dimples are arranged. Examples of the polyhedron to be used include a regular hexahedron, a regular octahedron, a regular dodecahedron, a regular icosahedron, a cube-octahedron, an icosa-dodecahedron and the like.
The regular octahedron has been admired for a general golf ball for a long time because dimples are systematically aligned finely. Twelve comparting lines obtained by projecting twelve sides of the regular octahedron form three great circles. These great circles are orthogonal to each other. The spherical surface is comparted into eight spherical regular triangles through the comparting lines (that is, three great circles). Dimples are arranged equivalently on the inside of each spherical regular triangle. The dimple does not intersect the twelve comparting lines. Accordingly, the three great circles do not intersect the dimple. Portions corresponding to the great circles act as great circle paths where the dimple is not present. By the existence of the great circle path, for example, there is an advantage that a directional alignment can easily be carried out before patting. Such a dimple arranging method is referred to as a regular octahedron pattern.
The golf ball is formed by upper and lower molds comprising semispherical cavities. A spew is generated in a portion (a so-called seam) corresponding to the parting lines of the upper and lower molds on the surface of the formed golf ball. The spew is ground and removed through a grindstone or the like. In an ordinary regular octahedron pattern, one of the three great circle paths is coincident with the seam. Consequently, the dimple is not present on the seam and the spew can easily be removed. Such a golf ball has been disclosed in Japanese Laid-Open Patent Publication No. Sho 60-11665 (1985/11665).
In the golf ball having the regular octahedron pattern, dimples are not present on the seam. Therefore, a dimple effect tends to be insufficient when the seam (to be the great circle path) is coincident with a portion in which a circumferential speed of backspin is the highest (which will be hereinafter referred to as the xe2x80x9chighest speed portionxe2x80x9d). As described above, the spew generated on the seam is removed by the grinding, so there is a possibility that the vicinity of the seam might be ground simultaneously and the dimples might be deformed, resulting in a reduction in the dimple effect. Furthermore, the dimple patterns on the right and left of the seam are equivalent to each other and the equivalent dimple patterns appear repetitively along the seam during the rotation of the golf ball. Therefore, the dimple effect tends to be insufficient when the seam is coincident with the highest speed portion. In the golf ball having the regular octahedron pattern, the following three unfavorable conditions are satisfied on the seam:
(1) the seam is a great circle path having no dimple;
(2) dimples provided around the seam might be deformed by cutting; and
(3) a dimple pattern appearing along the seam by rotation is monotonous.
The drawbacks (1) and (3) described above are caused when one of other great circle paths, as well as the seam, is coincident with the highest speed portion.
Japanese Laid-Open Patent Publication No. Hei 4-126166 (1992/126166) has disclosed a golf ball in which a regular octahedron pattern is based, a mold is devised and one great circle path is provided. Moreover, Japanese Laid-Open Patent Publication No. Hei 4-150875 (1992/150875) has also disclosed a golf ball in which a dimple pattern is similar to the regular octahedron pattern and one great circle path is provided. In these golf balls, it is possible to prevent the dimple effect from being reduced when a great circle other than the seam is coincident with the highest speed portion. However, when the seam is coincident with the highest speed portion, the drawbacks (1) to (3) are still caused. In these golf balls, moreover, one great circle path is provided so it is hard to obtain an original advantage of the regular octahedron pattern, that is, the easiness of a directional alignment before patting.
In consideration of the above-mentioned problems, it is an object of the present invention to provide a golf ball having three great circle paths and capable of preventing dimple effects from being reduced when the great circle path is coincident with the highest speed portion.
In order to achieve the above-mentioned object, the present invention provides a golf ball in which a surface thereof is comparted into eight spherical regular triangles through twelve comparting lines formed by projecting, onto the surface, twelve sides of a regular octahedron inscribed on the surface and dimples are arranged in the spherical regular triangles so as not to intersect any of the comparting lines, resulting in formation of three great circle paths,
wherein each of four spherical regular triangles sharing each of six apexes of the regular octahedron positioned on the surface has different dimple pattern from those of three other spherical regular triangles, respectively.
The expression of xe2x80x9cdifferent dimple patternsxe2x80x9d implies such a state that the dimples do not completely overlap each other even if the two spherical regular triangles to be compared overlap each other in any way. In the golf ball, since any of the comparting lines does not intersect the dimples, three great circle paths are present on the surface. Accordingly, it is possible to maintain an advantage of the regular octahedron pattern that the dimples are provided finely and the directional alignment of the golf ball can easily be carried out.
In the golf ball, also in the case in which any of the three great circle paths is coincident with the highest speed portion, the four spherical regular triangles appearing on the right side of the highest speed portion through the rotation of the golf ball have different dimple patterns from each other. Moreover, the four spherical regular triangles appearing on the left side of the highest speed portion through the rotation of the golf ball also have different dimple patterns from each other. In other words, the dimple patterns appearing through the rotation are not monotonous. Accordingly, it is possible to complement a reduction in the dimple effects caused by the great circle path. Consequently, the flight distance of the golf ball can be increased, and furthermore, flight performance can be prevented from being varied depending on a position of the highest speed portion.
It is preferable that all the eight spherical regular triangles should have dimple patterns which are rotational symmetrical. Consequently, if any of the three great circle paths is coincident with the highest speed portion, the individual spherical regular triangles can produce the dimple effects equivalently.
It is preferable that one to three of four spherical regular triangles sharing each of the six apexes of the regular octahedron should have dimple patterns which are rotational symmetrical and line symmetrical (hereinafter referred to as xe2x80x9crotational symmetrical/line symmetricalxe2x80x9d) and the other spherical regular triangles should have dimple patterns which are rotational symmetrical and are not line symmetrical (hereinafter referred to as xe2x80x9crotational symmetrical/non-line symmetricalxe2x80x9d). This state can also be achieved in any of the six apexes of the regular octahedron. More specifically, even if any of the great circle paths is coincident with the highest speed portion, the spherical regular triangle which is rotational symmetrical/line symmetrical and the spherical regular triangle which is rotational symmetrical/non-line symmetrical are mixed on the right side of the great circle path. Similarly, the spherical regular triangle which is rotational symmetrical/line symmetrical and the spherical regular triangle which is rotational symmetrical/non-line symmetrical are mixed on the left side of the great circle path. Consequently, the dimple patterns appearing through the rotation are not monotonous. It is the most preferable that two of the four spherical regular triangles sharing each of the six apexes should be rotational symmetrical/line symmetrical and two other spherical regular triangles are rotational symmetrical/non-line symmetrical.
It is preferable that each of the spherical regular triangles has 40 to 55 dimples arranged therein. Consequently, excellent dimple effects can be obtained and the flight performance of the golf ball can be enhanced.
In the eight spherical regular triangles, a difference between the number of dimples in the spherical regular triangle having the greatest number of dimples arranged therein and the number of dimples in the spherical regular triangle having the smallest number of dimples arranged therein is preferably four or less. Consequently, the aerodynamic symmetry of the golf ball can be enhanced.
It is preferable that one of the three great circle paths should be coincident with a seam to be a portion corresponding to a parting line of a pair of golf ball molds having semispherical cavities. Consequently, a spew can be removed easily. Since the great circle path is coincident with the seam, such a drawback that the dimple might not be present and surrounding dimples might be deformed through grinding is caused on the seam. However, the dimple pattern appearing through the rotation is not monotonous. Therefore, it is possible to prevent the dimple effects from being reduced when the seam is coincident with the highest speed portion.